My claim in this article is quite simple. Ideals perform a constitutive role in reasoning. By ‘ideals’ I mean conceptions of how things should be, sometimes hard to articulate and even harder to substantiate. By ‘constitutive’ I mean that without such conceptions we are not really reasoning, but merely acting according to some rules or standards of reasoning – which is not the same. The ‘constitutive role’ is not meant in the sense of being there before we develop a faculty of reasoning, or performing a guiding role in the acquisition of a set of roles. The role they exercise is in the everyday uses of reason, making decisions, giving meaning.The argument for this claim is as follows. Both in making decisions and giving meaning, we are performing motivated actions. The motivation, however, often leaves something to be desired. We are not able to fully explain why we did something, or what we mean by some expressions. Now this is not a sign of fallacious reasoning, but of the fact that reasons are underdetermined. They must be, because they are something we can disagree with. A law or a formula is not an argument. You can apply it or not. Of course, laws and formulas are immensely powerful tools for reasoning. But when employed as arguments, they merely function as discussion-stoppers. A formula is not an argument.Motivations take into account two aspects of things: how things are and how things should be. A motivation that does not take into account how things are is plainly ineffective. But a motivation that leaves things as they are motivates nothing. A solid motivation, then, relies on a conception of how things should be, but one that is under constant scrutiny in view of the facts. Without this scrutiny, it declines into the kind of ‘if only’-reasoning that is often denounced as ‘idealism’. In so far as this is a label for defective reasoning, I agree with the diagnosis but disagree with the name.The problem with ‘how things should be’ is that there is really no way to find out. (‘Things’ here is shorthand for life, reality, or any object intended in an action or expression.) As far as I’m concerned, it is okay to trace the origin of ideals in physical drives, empathic responses, and the spin-off from restless minds. They are certainly not ‘things’ in the sense of isolable objects; identifiable best, perhaps, by the words in which they are expressed. There is, then, no such thing as a ‘science of ideals’ but at best an analysis of how the terms that denote ideals are used and abused. Such an analysis will always be a scrutiny and often an unmasking; it comes closest to a ‘critique of ideology’ or the original meaning of idéologie.Moreover, a conception of how things should be prompts the question of why things aren’t that way already. If sufficiently clear and coherent to be formulated, conceptions of how things should be will often demand something impossible. But this, again, is not a defect. Soyez réaliste, démandez l’impossible – the slogan of the soixante-huitards sums it up neatly. A proper motivation demands something that comes not of itself. ‘How things are’ can be a reason not to something, but not a sufficient reason to do something. One way of identifying the conceptions underlying these motivations as ideals is precisely that they are supererogatory. In that sense, every ideal demands too much.